The present invention relates to a hydrometallurgical process for the separation, extraction and recovery of nickel and cobalt values from a sulfidic flotation concentrate. More specifically, the process involves an atmospheric pressure chlorine leach conducted under acidic conditions, followed by an oxidative acid pressure leach, and recovery of nickel by an electrowinning step.
Metallurgists have long sought to develop economically viable hydrometallurgical processes for the recovery of base metals from sulfidic flotation concentrates, as an alternative to the conventional smelting and refining processes. Smelters have become increasingly costly to build and to operate as a result of more stringent environmental emission controls. Many of the processes studied over the past fifty years have utilized the leaching of an aqueous slurry of the sulfidic flotation concentrate in an air or oxygen atmosphere in a pressure autoclave, to achieve the primary separation of the metal values from the iron, sulfur and gangue components of the concentrate. Few of these processes have achieved commercial success.
A hydrometallurgical process based on the pressure leaching of nickel-copper sulfide flotation concentrates in ammoniacal ammonium sulfate solution in an atmosphere of air at 95 C., was commercialized by Sherritt Gordon Mines Limited in Canada in the early 1950s. (J. R. Boldt, Jr. and P. E. Queneau, The Winning of Nickel, Longmans Canada Limited, Toronto, 1967, pp 299-314), and was operated successfully for forty years. Due to its relatively high energy consumption, and the necessity for providing a market for the ammonium sulfate fertilizer by-product, this process has not been widely adopted by other nickel producers because it was particularly adapted to the general location and available feedstocks of the Sherritt Gordon plant.
A sulfuric acid based pressure leaching process for the treatment of zinc sulfide flotation concentrates has also been in successful commercial operation since 1981 and has subsequently been adopted by several zinc producers to replace the conventional roasting technology. In this process, the zinc sulfide flotation concentrate is oxidatively pressure leached in a sulfuric acid solution at 150xc2x0 C., to produce a solution of zinc sulfate, and a residue consisting of elemental sulfur and iron oxide. The zinc sulfate solution is purified to remove trace impurity metals, and zinc metal is recovered from the purified leach solution by the long-established electrowinning process. Typically less than 10% of the sulfide in the concentrate is oxidized to sulfate in the pressure leach, with the balance being recovered as elemental sulfur in solid form, for sale or storage. The deportment of the sulfur in this process gives it a major advantage over the older roasting process, in which the sulfur is all converted to sulfuric acid, which must frequently be marketed at a loss, or converted to a solid waste such as gypsum for landfill disposal.
There have been numerous attempts to extend this oxidative acid pressure leaching technology to the direct treatment of copper and nickel-copper sulfide flotation concentrates, but no such process has as yet been successfully commercialized. A major obstacle to the application of oxidative pressure leaching at temperatures above the melting point of sulfur, to the treatment of chalcopyrite-containing flotation concentrates, has been the tendency of molten sulfur to coat the surface of the metal sulfide particles. This inhibits and prevents the reaction of the metal sulfide with the acid solution. Consequently, the oxidative pressure leaching of copper concentrates in sulfuric acid at temperatures above 120xc2x0 C. typically results in slow reaction rates and low metal extractions. Zinc and nickel sulfide particles have a weaker affinity for molten sulfur than do copper sulfides, and the successful zinc oxidative pressure leach process described above utilizes organic additives, such as lignosulfonate salts or quebracho, to prevent the coating of the sulfide particles during the leaching process. These additives are ineffective for copper sulfide concentrates, but recently, (see U.S. Pat. No. 5,730,776 to Collins et al.), the addition of low grade coals has been found to prevent the coating of both zinc and copper sulfides in oxidative pressure leaching in sulfuric acid solution at 150xc2x0 C.
A different approach to overcoming the problem of the occlusion of the sulfide particle surfaces by molten sulfur is described in U.S. Pat. No. 4,039,406 to Stanley et al. This patent discloses the addition of low concentrations of chloride ion to the leach solution in the oxidative pressure leaching of a chalcopyrite concentrate in sulfuric acid solution at temperatures above 120xc2x0 C. The benefits of the chloride addition were shown to be greatly increased rates of leaching of chalcopyrite and a major reduction in the amount of sulfide oxidized to sulfate, and as a result, the recovery of virtually all the iron content of the concentrate as hematite in the solid residue. In the absence of the chloride ion addition, oxidative pressure leaching of chalcopyrite in sulfuric acid at 150xc2x0 C. typically produces a leach solution containing high levels of acid and dissolved iron. With the chloride addition, the leach solution typically has a pH value of 2.5 to 3, and the iron concentration is less than 1 g/L. A further consequence of the low degree of sulfur oxidation, and the resulting low level of acidity in the chloride containing leach solution, is that a large portion of the leached copper can be reprecipitated as basic copper sulfate. The extent of this effect can be varied by adjusting the amount of acid added to the leach. This ability to control the deportment of the copper between solution and leach residue provides considerable flexibility in the design of the copper recovery process.
The applicability of oxidative pressure leaching with a chloride ion addition to a sulfate solution in the pressure leaching of nickel-containing sulfide flotation concentrates was subsequently described in a paper entitled xe2x80x9cOxygen Pressure Leaching of Fexe2x80x94Nixe2x80x94Cu Sulfide Concentrates at 110xc2x0 C.xe2x80x94Effect of Low Chloride Additionxe2x80x9d Subramanian et al. (Hydrometallurgy 2, (1976), pp. 117-125).
More recently, D. L. Jones in U.S. Pat. Nos. 5,431,788; 5,645,708; 5,650,057; 5,855,858; 5,874,055; and 5,902,474 discloses the combination of the chloride-assisted sulfuric acid oxidative pressure leaching of copper sulfide concentrates with the recovery of copper by a variety of process flowsheets based on conventional solvent extraction and electrowinning.
U.S. Pat. No. 5,650,057 issued to Jones, describes a hydrometallurgical process for the extraction of copper from an ore or concentrate. The process broadly comprises subjecting the ore or concentrate to an oxidative pressure leach in an acidic solution containing halogen ions and a source of bisulfate or sulfate ions. The process extends to the extraction of non-cuprous metals such as zinc, nickel and cobalt. Significantly, during the oxidative pressure leach the metal may be precipitated as an insoluble basic salt, such as basic copper sulfate, or substantially completely solubilized and precipitated later as the basic copper salt. The specific application of this process to nickel-cobalt containing sulfide concentrates is described in greater detail in U.S. Pat. No. 5,855,858, also issued to Jones.
Of interest to the present invention is the process illustrated in FIG. 1 of U.S. Pat. No. 5,855,858. The process involves subjecting a copper-nickel flotation concentrate to an oxidative pressure leach. Following separation of the leach solution and leach residue, the leach solution is passed to a copper solvent extraction circuit. The raffinate from the copper extraction step is then passed to a conventional nickel hydroxide precipitation step using lime. Following liquid/solid separation, the residue containing substantially the total nickel and cobalt values is subjected to an ammoniacal leach and the leach solution therefrom first passed to a solvent extraction circuit to remove cobalt. The raffinate from the cobalt extraction step is finally passed to nickel solvent extraction and electrowinning to thereby produce nickel cathode.
It is to be noted that in processes for the recovery of nickel from ores or concentrates, which include a chloride-assisted oxidative pressure leach in the process, heretofore it has been necessary that the copper-free nickel-containing leach stream undergo a solvent extraction processing step in order to produce a purified high strength nickel sulfate solution, suitable for recovery of the nickel values by electrowinning therefrom.
The benefits of finely grinding a base metal sulfide flotation concentrate prior to oxidative pressure leaching were first identified in the 1970s by Gerlach et al., (xe2x80x9cActivation and Leaching of Chalcopyrite. Concentrates in Dilute Sulfuric Acidxe2x80x9d, International Symposium on Hydrometallurgy, 1973, pp. 401-416), and Pawlek (xe2x80x98The Influence of Grain Size and Mineralogical Composition on the Leachability of Copper Concentratesxe2x80x99, International Symposium on Copper Extraction and Refining, 1976, pp. 690-705). At that time, the fine grinding of flotation concentrates to particle sizes below about 30 microns was considered to be uneconomical due to high energy consumption with existing milling technology. Significant advances in fine grinding technology have occurred over the past thirty years, and a variety of grinding mills are now commercially available to reduce the particle size of base metal flotation concentrates to less than 20 microns, without excessive consumption of energy. A number of hydrometallurgical processes have recently been proposed, which rely on fine grinding prior to the metal dissolution step in order to achieve high metal extraction under significantly milder leaching conditions, than are required for conventionally sized flotation concentrates. Examples of this type of process are described in U.S. Pat. Nos. 5,232,491 to Corrans et al., and 5,917,116 issued to Johnson et al. The U.S. ""491 patent teaches a method of activating a mineral species by fine or ultra fine milling thereof prior to processing by methods of oxidative hydrometallurgy. The U.S. ""116 patent teaches a method of processing a copper mineral by initially milling the mineral to a particle size of between 2 to 20 microns. The mineral is then oxidatively pressure leached in the presence of chloride ions at a temperature below the melting point of elemental sulfur.
Deleteriously, despite the advantages of the oxidative pressure leaching process using mixed chloride-sulfate solutions, the process flowsheets proposed by Jones in his series of the above-referenced U.S. patents for the treatment of nickel-containing ores or concentrates are relatively complex with numerous inefficient separation steps. They have yet to find commercial acceptance. Thus, there exists the need for a sulfidic nickel flotation concentrate treatment process which combines the advantages of an oxidative pressure leach in mixed chloride-sulfate solutions, with simpler and more direct solution purification and metal recovery processes.
It is a primary objective of the present invention to provide a novel hydrometallurgical process for the recovery of nickel and cobalt values from a sulfidic flotation concentrate, which process includes an atmospheric acid chlorine leach followed by an oxidative pressure leach, and wherein the purified nickel leach solution obtained therefrom may be directly passed to an electrowinning circuit to thereby produce nickel cathode. Furthermore, it is an important requirement that the dissolution of the metal values occurs rapidly and that a high recovery of sulfur as elemental sulfur be attained.
It is a secondary objective of the invention to provide, from the atmospheric acid chlorine leach, a chloride-containing feed solution suitable for use in the oxidative pressure leach whereby the primary objective described above may be attained.
It is yet a further objective of the invention to utilize the chlorine produced in the nickel electrowinning step as an oxidant in the leaching of the sulfidic flotation concentrate.
Furthermore, it is an objective of the invention to provide a process with the minimum number of process steps, and with minimum reagent costs.
Finally, as with all potential commercially viable hydrometallurgical processes, one seeks always to obtain maximum recovery of metal values, with minimum capital and operating cost.
In accordance with the present invention there is provided a process for the extraction, separation and recovery of nickel, cobalt, and copper from a nickel-cobalt-copper-sulfide containing flotation concentrate. The process comprises the steps of preparing a slurry containing the sulfidic flotation concentrate and contacting said slurry with an oxygen and chlorine-containing gaseous stream in an atmospheric acidic leach stage to thereby generate a pressure leach feed slurry containing a predetermined concentration of chloride ions therein. The pressure leach feed slurry is then subjected to an oxidative pressure leach under acidic conditions at elevated temperature to selectively leach the majority of the nickel, cobalt and copper therefrom to form a nickel, cobalt, copper-containing leach solution and a leach residue. The nickel-cobalt-copper-containing leach solution after separation of the leach residue therefrom, is treated to separately recover the copper and cobalt, and to remove impurities to thereby form a purified nickel leach solution. The nickel is then electrowon from said purified nickel leach solution to thereby produce nickel cathode and oxygen, chlorine and sulfuric acid therefrom. Preferably the sulfidic flotation concentrate is finely ground prior to the atmospheric acid chloride leach and most preferably to a particulate size ranging between about 10 to 30 microns. The preferred chloride concentration of the pressure leach feed solution would range between about 2 to 40 g/L.
In a preferred embodiment of the invention, there is broadly provided a process for the extraction, separation and recovery of nickel, cobalt and copper values from a nickel-cobalt-copper sulfide-containing flotation concentrate which comprises preparing a slurry containing flotation concentrate and contacting said slurry withan oxygen and chlorine-containing gaseous stream in an atmospheric acidic leach to thereby generate a pressure leach feed slurry containing a predetermined concentration of chloride ions therein. The pressure leach feed slurry is subjected to an oxidative pressure leach under acidic conditions at elevated temperature to selectively leach the majority of the nickel, cobalt and copper therefrom to form a nickel-cobalt-copper-containing leach solution and a leach residue. The nickel-cobalt-copper-containing leach solution is subsequently separated from said leach residue. The contained copper is removed from said nickel and cobalt-containing leach solution, and the copper-depleted nickel and cobalt-containing leach solution is neutralized and iron is removed therefrom. The nickel and cobalt containing leach solution is subjected to a purification step to thereby remove impurities, typically calcium, zinc and lead, from the nickel-cobalt containing leach solution. Cobalt is removed from said nickel, cobalt-containing leach solution utilizing solvent extraction and the nickel is extracted by electrowinning from said purified nickel-containing solution. Again, the preferred particulate size of the finely ground sulfidic flotation concentrate would be as described hereabove, as would the concentration of chloride ions in the pressure leach feed solution.
The invention further extends to a process for the preparation of a pressure leach feed slurry from a nickel-cobalt-copper sulfide-containing flotation concentrate comprising the steps of finely grinding said flotation concentrate to a preselected particulate size; preparing a slurry containing said finely ground flotation concentrate in an acidic solution and contacting said slurry with an oxygen and chlorine-containing gaseous stream to thereby generate a pressure leach feed slurry containing a predetermined concentration of chloride ions therein.
Additionally, the invention contemplates recycling the formed combined oxygen/chlorine stream generated in the nickel electrowinning step to the atmospheric acid chlorine leach. Also, it is particularly advantageous to recycle the sulfuric acid-containing nickel anolyte stream to the atmospheric acid chlorine leach step.
Beneficially, by first subjecting the sulfidic flotation concentrate to an atmospheric acid chlorine leach, prior to the oxidative pressure leach, the rate of oxidation of the sulfide minerals is increased, thereby significantly reducing the size and cost of the autoclaves required for the oxidative pressure leach process.
Advantageously, as a result of practicing the process of the instant invention it is possible to directly electrowin nickel from the purified chloride-containing high strength nickel sulfate leach solution derived from the oxidative pressure leach, to thereby produce nickel with the consequent elimination of the need for nickel upgrading and purification steps involving intermediate precipitation, redissolution and solvent extraction. Furthermore, as a result of practicing the electrowinning of nickel from a chloride-containing nickel sulfate solution, in combination with the recycle of the gas stream produced at the anode, to the atmospheric leach, the chlorine is recovered in the desired form of dissolved chloride ion, thereby eliminating the necessity of reacting the recovered gas stream with hydrogen to regenerate hydrochloric acid.
It is an essential requirement of the process of the invention that an atmospheric acid chlorine leach be conducted on the sulfidic flotation concentrate prior to the oxidative pressure leach.
The recovery of nickel is improved and the reaction time in the oxidative pressure leach is markedly reduced in a relatively uncomplicated, commercially viable process involving fewer process steps than those taught by the prior art.
Without being bound by same, it is believed that the novel incorporation of an atmospheric acid leach step, in the presence of chlorine as the oxidant, prior to the conduction of the oxidative pressure leach, is instrumental in the achievement of the objectives described above.
As an additional advantage, the provision of a fine grinding step of the sulfidic flotation concentrate prior to the atmospheric acid chlorine leach step assists in increased metals recovery.